“I train 24 hours a day,” the sprinter told PEOPLE ahead of the games. “Training isn’t just physical out on the track. I’m on the track maybe two hours, I’m in the gym maybe an hour and a half, but mentally, I’m always locked in. It’s all day.”

Merritt won gold in the 400-meter at the Beijing Games, but in October 2010 he was suspended for failing a drug test.

He planned to make his Olympic return in 2012, but with the hamstring injury, he failed to qualify for the 400-meter final.

For more of PEOPLE’s Olympic coverage, pick up our collector’s edition,The Best of the Games, on sale now.

For more of PEOPLE’s Olympic coverage, pick up our collector’s edition,The Best of the Games, on sale now.

“Rest, recovery and work hard,” Merritt told PEOPLE of his schedule leading up to Rio.

The athlete returns to the track on Tuesday to compete in the men’s 200-meter dash, where he faces international superstars like van Niekerk, Justin Gatlin and Usain Bolt.

Merritt is also expected to anchor the 4×400-meter relay on the final evening of the Olympics.